Required for a domestic fuel cell system are lowering heating and electricity charges and improving an effect of reducing carbon dioxide by a DSS (Daily Start & Stop or Daily Start-up & Shut-down) operation in which the fuel cell system operates during daytime and stops during nighttime. In accordance with the DSS operation, the fuel cell system frequently stops operating. In a case where a conventional fuel cell carries out such operation, an electrode catalyst deteriorates by, for example, a reactant gas remaining in a cell stack or air intruding from outside when the fuel cell is not generating electric power. This causes a problem of deterioration of a cell performance.
As a fuel cell system capable of dealing with such problem, Patent Document 1 proposes a fuel cell system configured such that while the fuel cell system stops generating the electric power, supply and discharge of each of a fuel gas and an oxidizing gas are stopped, and an inactive gas is injected into a fuel gas passage and oxidizing gas passage of the fuel cell. In the foregoing, the fuel gas passage denotes a passage through which the fuel gas is supplied to the fuel cell and a passage through which the fuel gas is discharged from the fuel cell, and the oxidizing gas passage is a passage through which the oxidizing gas is supplied to the fuel cell and a passage through which the oxidizing gas is discharged from the fuel cell.
Patent Document 2 proposes a fuel cell system configured such that: the fuel gas passage and the oxidizing gas passage are closed when the fuel cell system is not generating electric power; the inactive gas is supplied to a fuel gas filled space which is practically isolated from outside by closing the passages and is constituted by the fuel gas passage in the fuel cell and a space communicated with the fuel gas passage; and the air is supplied to an oxidizing gas filled space which is practically isolated from outside by closing the passages and is constituted by the oxidizing gas passage in the fuel cell and a space communicated with the oxidizing gas passage. Generally, each of the pressure in the fuel gas filled space and the pressure in the oxidizing gas filled space decreases by the consumption of the gas due to, for example, cross leakage or by temperature decrease. However, in accordance with the fuel cell system described in Patent Document 2, to prevent the pressure in the fuel gas filled space and the pressure in the oxidizing gas filled space from decreasing, the inactive gas is supplied to the fuel gas filled space, and the air is supplied to the oxidizing gas filled space. In the oxidizing gas filled space, since oxygen is consumed and only nitrogen remains, an electrode potential can be suppressed to a low level. In addition, since the gases are supplied to the inside of the fuel cell to compensate pressure decrease, there is no pressure difference between the inside of the fuel cell and the atmosphere. Therefore, the intrusion of oxygen into the fuel gas filled space is suppressed to an extremely low level, and the increase in the electrode potential is more effectively suppressed.
Patent Document 1: Japanese Laid-Open Patent Application Publication 2005-222707
Patent Document 2: International Publication WO 2007/063826